Phase-locked loops are often used as clock regenerators or frequency synthesizers in digital systems. By including multiple output dividers, a phase-locked loop with a single VCO output frequency may act as a frequency synthesizer that generates multiple output frequencies. For example, FIG. 1 illustrates a multiple-output phase-locked loop frequency synthesizer 100. Frequency synthesizer 100 synthesizes five output signals 105 having frequencies f1 through f5, respectively. Depending upon the frequency division provided by a corresponding output divider 110 for each output signal 105, each of the frequencies f1 through f5 is a fraction of the output frequency of a voltage-controlled oscillator (VCO) 115. One of the output signals 105 is selected by a multiplexer 140 for use as a feedback signal, divided by a feedback divider 145, and received at a phase detector 150. Similarly, phase detector 150 also receives a divided version of an input signal 130 after processing by an input divider 155. A loop filter 160 receives the output of phase detector 150 and drives VCO 115. In this fashion, depending upon the settings of input divider 155, feedback divider 145, output dividers 110, and multiplexer 140, the synthesis of frequencies f1 through f5 is kept coherent with the frequency of input signal 130.
Although operation of frequency synthesizer 100 is relatively straightforward, configuring it to provide a desired set of output frequencies is not. Instead, considerable preparation is required in the configuration of the various dividers, the selection of the feedback signal, and the selection of a candidate VCO output frequency to provide the desired output frequencies based upon an input reference frequency of an input signal 130. For example, if each of the seven dividers 155, 145, and 110 in frequency synthesizer 100 has 32 settings (5 bit selection), these dividers alone provide 327 different configurations. Taking into account the additional freedom one has in choosing a feedback signal from the five output signals 105, frequency synthesizer 100 may be configured over one hundred billion different ways. Given such a large search space, it is impractical to search for optimal solutions through a brute force application. Moreover, the non-linear interaction of the various dividers prevents easy convergence to an optimal solution in an analytical approach.
Accordingly, there is a need in the art for improved techniques to configure multiple-output frequency synthesizers.